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Let's consider a molecule with N atoms, on which is shot a polychromatic IR beam. From the quantum selection rules, we know that in a single event of absorption only one of the 3N-6 oscillators can be excited or disexcited of one level. If we assume the oscillator in the ground state, then it absorbs from the radation a photon with a proper energy and goes in the second level.
I also have studied that in Raman effect, the visible radiation, which is more energetic than IR, excites the oscillator in a virtual state and then the oscillator relaxes down on the ground state or on another excited level by emetting a photon with energy corresponding to the jump.
So I don't understand why when an oscillator absorbs an IR photon then it doesn't relases it and comes back to its orginal state? I mean it is what happens during the Raman effect, why it doesn't happen also with IR? In this case I should not see any IR spectra since the same photon is absorbed and then emitted.

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  • $\begingroup$ Raman scattering happens in the IR as well, so I’m unclear on just what you are asking about? For instance, you can use stimulated Raman to generate a 1.8 micron coherent beam from Nd:YAG in hydrogen. $\endgroup$ – Jon Custer Dec 25 '18 at 22:35
  • $\begingroup$ I'm just asking how is possbile to see IR spectra if, like it happens in Raman scattering, the oscillator after been excited relases a photon of the same energy when it comes back on the ground? I mean on the detector should arrive all the photons of the initial beam...@JonCuster $\endgroup$ – Landau Dec 26 '18 at 9:14
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Maybe it will be helpful to refer to the Einstein $A$ and $B$ coefficients.

The bottom line is that absorption and stimulated emission do both occur. The appropriate rate coefficients ($B$) for the upward and downward processes are actually equal.

The reason that net absorption is seen is that the population of the ground state is larger than the excited state. Here, I am assuming thermal equilibrium: one can imagine situations in which this assumption is violated, but I believe the question relates to the usual, equilibrium situation.

Spontaneous emission also occurs, at a rate governed by the $A$ coefficient. One can relate $A$ and $B$ by using a detailed balance argument; that's not critical to the present discussion, but it is explained on that Wikipedia page. The main point is that we only see a tiny fraction of those photons, the ones that happen to be emitted directly towards us, because the spontaneous emission occurs isotropically. So, even in an idealized picture where a steady state is maintained by equal rates of absorption and emission (spontaneous + stimulated) with no other relaxation processes, you would still expect to see an absorption line in the spectrum.

I haven't mentioned the Raman scattering. One can discuss it in terms of a "virtual state", but I think it's misleading to draw too close an analogy to the processes of IR absorption and emission. I mean, it isn't really absorption followed by emission. It is more like inelastic scattering of a photon.

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